1. Field of the Invention
The present invention relates to an optical head and an optical recording and/or reproducing apparatus, and more particularly, to an optical head adapted use an evanescent light from the end face of a solid immersion lens disposed opposite to a recording medium to write and/or read a signal to and/from the recording medium, and an optical recording and/or reproducing apparatus using the optical head.
2. Description of the Related Art
Referring now to FIG. 1, there is schematically illustrated the construction of a magneto-optical disc (will be referred to as xe2x80x9cMO discxe2x80x9d hereinafter) as a recording medium to and/or from which data is magneto-optically written and/or read. The MO disc is generally indicated with a reference 100. As shown, the MO disc 100 comprises a substrate 101, a first dielectric layer 102 formed on the substrate 101 from SiN or the like, a magnetic layer 103 formed on the first dielectric layer 102 from TbFeCo or the like, a second dielectric layer 104 formed on the magnetic layer 103 from SiN or the like, and a light-reflective layer 105 formed on the second dielectric layer 104 from Al or the like. The first dielectric layer 102, magnetic layer 103, second dielectric layer 104 and light-reflective layer 105 form together, a magneto-optical recording multilayer 106 (will be referred to as xe2x80x9cMO multilayerxe2x80x9d hereinafter). The MO multi-layer thin film 106 has formed thereon a protective layer 107 of an ultraviolet-curable resin or the like.
The MO disc 100 has written thereto a signal as a magnetized direction of the magnetic layer 103. For write and/or read of the signal to and/or from the MO disc 100, a laser light is irradiated from the substrate 101 towards the MO multilayer 106 as shown in FIG. 1.
Referring now to FIG. 2, there is schematically illustrated an example of the conventional optical head used to write and/read a signal to and/or from the above-mentioned MO disc 100. Note that the optical system for focusing servo and tracking servo is not shown in FIG. 2 for the simplicity of illustration and explanation of the optical head. The optical head is generally indicated with a reference 120.
For reading a signal recorded in the MO disc 100 with the aid of the optical head 120 shown in FIG. 2, a laser light is emitted from a laser source 121. It is guided through a collimator lens 122 and beam splitter 123 to be incident upon an objective 124. The laser light incident upon the objective 124 has been linearly polarized as shown in FIG. 3. The laser light incident upon the objective 124 is focused on the MO multilayer 106 of the MO disc 100 through the objective 124.
The light focused on the MO multilayer 106 of the MO disc 100 is reflected by the MO multilayer 106. At this time, the reflected light is changed in polarization state under the polar Kerr effect of the magnetic layer 103, as will be seen from FIGS. 4 and 5.
Note that the magnetized direction of the magnetic layer 103 is represented by a non-diagonal component ∈xy of dielectric tensor. FIG. 4A shows a dielectric tensor 6 f the magnetic layer 103, given by a following expression (1-1). FIG. 4B shows the polarized direction of the reflected light.                     (                                                            ϵ                xx                                                                    ϵ                xy                                                    0                                                                          -                                  ϵ                  xy                                                                                    ϵ                xx                                                    0                                                          0                                      0                                                      ϵ                xx                                                    )                            (1-1)            
FIG. 5A shows a dielectric tensor of the magnetic layer 103 whose magnetized direction is opposite to that shown in FIG. 4A, given by a following expression (1-2). FIG. 5B shows the polarized direction of the reflected light.                     (                                                            ϵ                xx                                                                    -                                  ϵ                  xy                                                                    0                                                                          ϵ                xy                                                                    ϵ                xx                                                    0                                                          0                                      0                                                      ϵ                xx                                                    )                            (1-2)            
As seen from FIGS. 4 and 5, the polarized direction of a return light from the MO multilayer 106 back to the objective 124 is changed depending upon the magnetized direction of the magnetic layer 103. As shown in FIG. 2, the return light passes through the objective 124 gain and is incident upon the beam splitter 123 which reflects the return light which will thus be taken out.
The return light reflected by the beam splitter 123 and taken out is first incident upon a half-wave plate 125 by which the polarized direction of the return light is rotated 45 deg. as shown in FIG. 6. Note that FIG. 6 shows the polarized direction having been rotated clockwise under the effect of polar Kerr effect of the magnetic layer 103 as shown in FIG. 5.
Next, the return light is incident upon a polarizing beam splitter 126 which will split t into two polarized components whose polarized directions are orthogonal to each other. The polarized component having been transmitted through the polarizing beam splitter 126 will be detected by a first photodetector 127, while the polarized component having been reflected by the polarizing beam splitter 126 will be detected by a second photodetector 128.
Referring now to FIG. 7, there is illustrated how the polarized light is split by the polarizing beam splitter 126. The polarization state of the light incident upon the polarizing beam splitter 126 is in two kinds. One is a case A that the polarized light returns after having the polarized direction thereof rotated through an angle xcex8k as shown in FIG. 7 counterclockwise depending upon the magnetized direction of the magnetic layer 103, and the other is a case B that the polarized light returns after having the polarized direction thereof rotated through an angle xcex8k clockwise depending upon the magnetized direction of the magnetic layer 103. Note that in FIG. 7, the I-axis corresponds to the polarized component transmitted through the polarizing beam splitter 126 and the J-axis corresponds to the polarized component reflected by the polarizing beam splitter 126.
More particularly, the light transmitted through the polarizing beam splitter 126 (namely, the light detected by the first photodetector 127) is a projection of the polarized light beams indicated with references A and B, respectively, onto the I-axis as in FIG. 7, and the light reflected by the polarizing beam splitter 126 (namely, the light detected by the second photodetector 128) is a projection of the polarized light beams indicated with references A and B, respectively, onto the J-axis as in FIG. 7. Thus, in the case of the polarized light beam A, J greater than I, and in the case of the polarized light beam B, J less than I. A magneto-optical signal (will be referred to as xe2x80x9cMO signalxe2x80x9d hereinafter) indicative of a magnetized direction of the magnetic layer 103 is detected as a difference (|I|2xe2x88x92|J|2) between an intensity of the polarized light detected by the first photodetector 127 and an intensity of the polarized light detected by the second photodetector 128.
In the magneto-optical disc system, the recording density can effectively be increased by focusing a laser light used for write and/or read of a signal through an objective having a larger numerical aperture (NA) which will lead to a smaller diameter of a light spot focused by the objective and thus to a higher resolution.
The diameter of the light spot focused by the objective is generally expressed by xcex/NA where xcex is a wavelength of a laser light used for write and/or read and NA is a numerical aperture of the objective. Also, the numeral aperture (NA) of the objective is expressed by nxc2x7sin xcex8 where n is a refractive index of a medium and xcex8 is an angle of marginal light incident upon the objective. Therefore, when the medium is air (that is, n=1), the NA of the objective cannot exceed 1.
For an NA larger than 1, an optical head has been proposed in which a solid immersion lens (will be referred to as xe2x80x9cSILxe2x80x9d hereinafter) is used as an objective. The SIL is supported opposite to an MO disc with a space between them, the space being smaller than the wavelength of a light used for write and/or read of a signal to and/or from the MO disc. The optical head using the SIL is adapted such that a collimated light beam is incident upon the SIL and the majority of the incident light beam is totally reflected at the end face of the SIL. An evanescent light leaking from the end face of the SIL is used for write and/or read of a signal to and/or from the MO disc. If the SIL uses therein a medium whose refractive index is n greater than 1, the NA can be made larger than 1.
Note that in the optical head using the SIL, the collimated light beam is incident upon the SIL such that the majority of the incident light beam is totally reflected at the end face of the SIL as mentioned above and the evanescent light leaking from the end face of the SIL reaches the MO disc. Therefore, the central light and marginal light of a collimated light beam incident upon the SIL will be incident upon the MO disc at different angle, respectively. The angle of the central light upon the MO disc is greatly difference from that of the marginal light.
When the SIL is used in the optical head, it is optically desirable that there should not exist no space between the SIL and MO disc. Nevertheless, some space should be between the SIL and MO disc since the MO disc has to be driven to spin at a high speed during write and/or read of a signal to and/or from the MO disc. However, the evanescent light leaking from the end face of the SIL will exponentially be attenuated as it goes away from the SIL end face. Therefore, for the evanescent light to sufficiently reach the MO disc, the space between the SIL and MO disc has to be sufficiently narrower than the wavelength of a light used for write and/or read of a signal to and/or from the MO disc.
For the aforementioned reasons, in case an SIL is used in the optical head, there will unavoidably be provided between the SIL and MO disc an air layer whose thickness is smaller than the wavelength of a light used for write and/or read of a signal to and/or from the MO disc. Therefore, the MO disc may be considered to have a, multi-layer optical thin film opposite to the SIL and which includes the air layer as well as the MO multilayer. That is, an incident light upon the SIL will be reflected by the multi-layer optical thin film including the MO multilayer and air layer.
Generally, when an incident light is reflected by the multi-layer optical thin film including the air layer, the phase difference and reflectance of the reflected light will vary depending upon the polarized direction, angle of incidence, etc. of the incident light. Thus, in an optical head using an SIL as an objective, a light returned, by reflection, from the multi-layer optical thin film including the air layer will unevenly be distributed when polarized. The uneven distribution of the polarized beams will cause the signal-to-noise ratio (SNR) to be worse especially when the MO signal is detected, for the MO signal is detected though detection of a polarized direction of the light returned, by reflection, from the multi-layer optical thin film
As in the foregoing, although the employment of the SIL permits to use an objective having a larger NA in an optical head, it gives birth to a new problem that the light returned, by reflection, from the multi-layer optical thin film including the air layer will unevenly be distributed when polarized.
Accordingly, the present invention has an object to overcome the above-mentioned drawbacks of the prior art by providing an optical head using a solid immersion lens as an object and capable of detecting a quality signal from a light returned, by reflection, from a recording medium by compensating the above-mentioned uneven distribution of polarized light. It is another object of the present invention to provide an optical recording and/or reproducing apparatus using the optical head.
The above object can be attained by providing an optical head comprising according to the present invention:
a solid immersion lens supported opposite to a recording medium with a space between them, the space being smaller than the wavelength of a light used for write and/or read of a signal to and/or from the recording medium, an evanescent light from the end face of the solid immersion lens being used for write and/or read of a signal to and/or from the recording medium; and
an optically anisotropic optical element formed not flat at at least one side thereof and disposed in the optical path of the light used for write and/or read of a signal to and/or from the recording medium.
The other object can be attained by providing an optical recording and/or reproducing apparatus using an optical head for write and/or read of a signal to and/or from a recording medium, the optical head comprising according to the present invention:
a solid immersion lens supported opposite to a recording medium with a space between them, the space being smaller than the wavelength of a light used for write and/or read of a signal to and/or from the recording medium, an evanescent light from the end face of the solid immersion lens being used for write and/or read of a signal to and/or from the recording medium; and
an optically anisotropic optical element formed not flat at at least one side thereof and disposed in the optical path of the light used for write and/or read of a signal to and/or from the recording medium.
In the above-mentioned optical head and recording and/or reproducing apparatus according to the present invention, during write and/or read of a signal to and/or from the recording medium, an uneven distribution of polarized light included in a light returned; by reflection, from the recording medium can be compensated by the optical element. Therefore, according to the present invention, the solid immersion lens is adopted and a quality signal can be detected from the return light from the recording medium by compensating the uneven distribution of polarized light.
These objects and other objects, features and advantages of the present intention will become more apparent from the following detailed description of the preferred embodiments of the present invention when taken in conjunction with the accompanying drawings.